Shares of Alliant Energy Corp. closed at an all-time high Friday after Wisconsin energy regulators endorsed a plan that will keep its base electricity rates unchanged for two more years.

The company's Madison-based utility, Wisconsin Power & Light Co., will see no change in base electricity rates through the end of 2014 under a plan the state Public Service Commission approved Friday.

The Madison utility offered the freeze as a way to provide customer certainty about their electricity bills at a time when the economy is still recovering.

Members of the Public Service Commission endorsed the plan, which also would result in a drop in local charges for its natural gas customers in 2013. Natural gas charges would then be frozen in 2014.

The downturn hit WP&L harder than other state utilities because it serves Rock and Sheboygan counties, which were hit hard by factory closings, including the shutdown of the Janesville General Motors Corp. plant.

That downturn in electricity sales led WP&L to increase rates several years ago to help cover its costs. Now the utility is saving money because of lower customer demand for energy conservation projects and its decision to no longer buy power from the Kewaunee nuclear power plant east of Green Bay.

Alliant Energy's shares closed at $45.50, up 39 cents or 0.86%. The shares have gained more than 15% over the past year.

Utilities are seeking multiyear rate settlements to give assurances to investors that they will be able to continue to earn profits at current levels despite an industry trend to ratchet down the profit levels earned by regulated utilities, analyst David Parker of Robert W. Baird & Co. said in a research note this week.

WP&L will be allowed to continue to earn a return of 10.4%, and will even be able to earn slightly more than that under the plan adopted. If it earns more, it will have to share a portion of the extra earnings with its customers, the PSC decided.